1. Field of the Invention
The present invention relates to an EUV light production apparatus that is used as a light source for an exposure system and so on, and particularly, to an apparatus for and a method of withdrawing ions emitted from a target that has been made in a plasma state at an EUV light production point.
2. Description of Related Art
A light lithography technique, in which a circuit pattern is optically transferred on a semiconductor wafer, is important for realizing the integration in an LSI. As an exposure system used for light lithography, one that employs a reduced projection exposure method, i.e. a stepper, is utilized at present. Specifically, a light transmitted through an original image (reticule) pattern irradiated with a light source is projected on a light-sensitive material on a semiconductor substrate via a reduced projection optical system to form a circuit pattern. The resolution of the projected image is limited in accordance with the wavelength of the light source. For this reason, with a demand for making the width of lines of a pattern finer, the wavelength of the light source is getting shorter toward an ultraviolet region.
Recently, KrF excimer laser (wavelength: 248 nm) and ArF excimer laser (wavelength: 193 nm), which oscillate to produce deep ultraviolet light (DUV light), are used as a light source, and F2 laser, which oscillates to produce vacuum ultraviolet light (VUV light), is developed.
At present, in order to realize a finer process, attempts are made to use an extreme ultraviolet (EUV) light source (wavelength: 13.5 nm), which outputs EUV light, as a light source for light lithography.
There is a laser production plasma (LPP) method as one of methods for producing the EUV light.
With an EUV light source employing the LPP method, a target is irradiated with short-pulse laser light so that the target is excited into a plasma state in which the EUV light is produced, and then the produced EUV light is collected by a collector mirror to be output.
FIG. 1 is a view conceptually showing a configuration of an EUV light production apparatus employing the LPP method and used as a light source for an exposure system.
A collector mirror 3 for collecting EUV light is provided inside a vacuum chamber 2. The EUV light collected by the collector mirror 3 is transmitted to an exposure system (not shown) provided outside the vacuum chamber 2. The exposure system is a system for forming semiconductor circuit patterns on a semiconductor wafer with the EUV light.
A vacuum is drawn on the inside of the vacuum chamber 2 by means of a vacuum pump or the like to evacuate the inside because the EUV light having a short wavelength of 13.5 nm is not effectively transmitted if not under vacuum.
A target 1 serving as a EUV light production source is located on a predetermined EUV light production point A in the vacuum chamber 2, namely, a condensing point of laser light. Tin Sn, lithium Li, xenon Xe, or the like is used as a material for the target 1.
A driver laser unit 4 serving as a laser oscillator performs pulse-oscillation to produce and emit laser light L. Nd:YAG laser, CO2 laser, or the like is used as a laser.
The laser light L via a laser condenser system is condensed on the EUV light production point A. The target 1 is irradiated with the laser light L at the timing when the target 1 reaches the EUV light production point A. The irradiation of the laser light L onto the target 1 makes the target 1 excited into a plasma state so that the target 1 emits EUV light.
The emitted EUV light diverges in all the directions centered on the plasma. The collector mirror 3 is disposed so as to surround the plasma. The EUV light that diverges in all the directions is collected by and reflected on the collector mirror 3. The collector mirror 3 selectively reflects the EUV light having a desired wavelength of 13.5 nm. The EUV light (output EUV light) reflected on the collector mirror 3 is transmitted to an exposure system.
The plasma emits neutral particles and ions having various velocities.
Meanwhile, with the demand for higher EUV light output, it is required to employ a high output laser unit as the driver laser unit 4 while high and stable output of the EUV light is maintained for a long period of time.
The neutral particles and ions emitted from the plasma are, however, deleterious in terms of the durability of the EUV light production system and the efficiency of the light output.
Specifically, since the collector mirror 3 is disposed in the vicinity of the plasma, the neutral particles and low-speed ions emitted from the plasma adhere to a reflection plane of the collector mirror 3, causing a deterioration in the index of reflection of the collector mirror 3.
On the other hand, high-speed ions emitted from the plasma damage multilayered film formed on the reflection plane of the collector mirror 3. This is called “spattering.”
The production of the neutral particles can be suppressed by using a target having a minimum mass as disclosed in International Publication No. WO2002/46839 pamphlet, page 1, or by producing a completely ionized plasma by means of double pulse irradiation or the like.
The production of ions, however, is inevitable so long as the plasma is produced so that measures against the ions are indispensable.
As described above, low-speed ions adhere to the collector mirror 3, and deteriorate the index of reflection of the collector mirror 3. The ions that have adhered to the collector mirror 3, however; can be removed in principle by cleaning with reacted gas or the like as disclosed in US2006/0091109A1. After the cleaning, the index of reflection of the collector mirror 3 recovers, and hence the collector mirror 3 may be continuously used.
However, for the EUV light production apparatus used for exposure, recently there has been a demand to prolong, up to at least one year, the period when the 10% deterioration occurs in the index of reflection of the collector mirror 3. To meet the demand, the allowed value of the adhesion amount (thickness) of a metallic film on the reflection plane of the collector mirror 3 is very small value, e.g. about 0.75 nm if the target 1 is made of tin Sn. The high rate and high speed of cleaning are therefore required.
On the other hand, the high-speed ions spatter the surface of the collector mirror 3 as described above to damage the reflection film of the collector mirror 3, resulting in a deterioration in the index of reflection of the collector mirror 3. Replacement of the collector mirror 3 is required when the collector mirror 3 is damaged and the index of reflection thereof deteriorates. There is a technique of reproducing the reflection film of the collector mirror 3 of the EUV light production apparatus. In such a technique, however, a coating unit for precisely carrying out a coating process with good evenness of surface, e.g. about 0.2 nm (rms) is additionally provided, which leads to the increase in the cost of the apparatus. Also, since the damage in the reflection film of the collector mirror 3 varies from place to place, it is substantially impossible to reproduce the reflection film having even dispersion of the index of reflection. For this reason, it is general to laminate reflection films having hundreds of layers on the collector mirror 3 in order to increase the lifetime until the replacement of the collector mirror 3.
Further, as a method for reducing the damage density due to the high-speed ions, the distance between the collector mirror 3 and the EUV light generation point A may be set longer. According to this method, however, the collection solid angle of the collector mirror 3 for collecting the EUV light becomes small, which may cause a problem that effective EUV output for exposure becomes low.
In order to solve the above problem, the collector mirror 3 having a large diameter, e.g. 500 mm or more may be used. However, if the collector mirror 3 having such a large diameter is manufactured, it is difficult to produce the reflection films having hundreds of layers white keeping the accurate surface roughness and geometry. Further, even if the collector mirror 3 as described above could be manufactured, the manufacturing process would become complicated and the time required for it would become long, resulting in the collector mirror 3 being very expensive.
Meanwhile, the radiation intensity distribution of the EUV light is dependent upon a laser light incidence direction. Specifically, the relatively strong EUV light is emitted in the direction opposite to the laser light incidence direction. There is therefore proposed a system configuration in which the collector mirror 3 is provided with a hole for the laser light to pass therethrough, and the laser light is emitted toward the target 1 via the hole, whereby the strong EUV light is effectively collected by the collector mirror 3 disposed at a place where the collector mirror 3 faces a direction opposite to the laser light incidence direction. It should be noted that the term “laser light incidence direction” herein is used to mean the direction of the laser light when the laser light is emitted onto the EUV light production point A.
Furthermore, it was recently found out that not only the above mentioned radiation intensity distribution of the EUV light but also the quantity and intensity (kinetic energy) distribution of the ions emitted from the plasma are dependent upon the laser light incidence direction. That is, as shown in FIG. 2, the quantity and intensity (kinetic energy) of ions emitted are large and strong (high) in the direction opposite to the laser light incidence direction. Accordingly, if the collector mirror 3 is disposed at a place where the collector mirror 3 faces the direction opposite to the laser light incidence direction as shown by an alternate long and short dash line in FIG. 2, a problem may occur that the collector mirror 3 is vigorously damaged and the lifetime thereof becomes extremely short.
In order to solve the above problem, as shown in FIG. 3, Japanese Patent Application Laid-open No. 2005-197456 discloses that, by applying a magnetic field to the ions emitted from the plasma by means of magnets such that magnetic lines of force are in the direction perpendicular to the laser light incidence direction, the ions emitted from the plasma are deflected to the above perpendicular direction not to reach the collector mirror 3, thereby preventing the ion adhesion and the spattering on the collector mirror 3.
As to the invention disclosed in the Japanese Patent Application Laid-open No. 2005-197456, however; several Ts of strong magnetic field are required to deflect the high-speed ions reaching 10 keV in order to prevent them from reaching the collector mirror 3, and accordingly the cost of the electromagnet for applying the magnetic field becomes high. As a result the total cost of the EUV light production apparatus becomes high.